1. Field of the Invention
The present invention relates to a highly sensitive, small-sized and inexpensive physical quantity sensor for reducing disturbance strain caused by temperature and a method for manufacturing the physical quantity sensor.
2. Description of Related Art
Traditionally, a pressure sensor that detects a pressure according to a change in capacitance between electrodes (for example, see Document 1: JP-A-10-082709) and a pressure sensor that detects a pressure according to a strain (for example, see Document 2: JP-A-2006-250837) are known as a physical quantity sensor.
FIG. 1 is an exploded perspective view of a pressure sensor disclosed in Document 1 and FIG. 2 is a cross section of the pressure sensor taken along II-II line.
As shown in FIGS. 1 and 2, a capacitive pressure sensor 10 includes: a conductive movable electrode 20; a first substrate 30 and a second substrate 40 that are glass substrates bonded to a thick portion 21 surrounding the movable electrode 20; and a pair of detection electrodes 31 provided on the first substrate 30.
In the above pressure sensor 10, the movable electrode 20 provided in a gap between the first substrate 30 and the second substrate 40 is bended by a pressure applied through a pressure inlet 41 provided in the second substrate 40.
Accordingly, capacitance varies between the movable electrode 20 and the pair of detection electrodes 31 provided on the first substrate 30 and the varied capacitance is electrically processed to measure the pressure.
On the other hand, a strain gauge pressure sensor as disclosed in Document 2 includes a semiconductor pressure-detecting element on the movable electrode 20 in place of the detection electrodes 31 and the like in FIGS. 1 and 2, where pressure is measured according to a change in resistance of the semiconductor pressure-detecting element.
In Document 2, the first substrate 30 and the second substrate 40 are prepared by a material having different coefficients of thermal expansion, thereby reducing nonlinear disturbance strain caused by changes in ambient temperature and static pressure.
When the typical pressure sensor 10 as disclosed in Documents 1 and 2 is manufactured, the thick portion 21 surrounding the movable electrode 20 is bonded to the first substrate 30 and the second substrate 40.
A known bonding method such as anodic bonding is used for bonding. In such a bonding, the movable electrode 20, the first substrate 30 and the second substrate 40 are heated to a high temperature (for example, approximately 400 degree C.).
When the pressure sensor 10 heated by the bonding is cooled down to a room temperature, there is a possibility that the movable electrode 20 may be bended due to a difference in the coefficients of thermal expansion between the movable electrode 20 and the first and second substrates 30 and 40.
Moreover, there is a possibility that the movable electrode 20 is drawn toward the first substrate 30 or the second substrate 40 by an electrostatic force and is bonded while the movable electrode 20 is bended.
When the movable electrode 20 is bended, particularly in the capacitive pressure sensor 10 a distance and a capacitance between the movable electrode 20 and the detection electrodes 31 vary to deteriorate temperature characteristics.
Further, there is a possibility that the movable electrode 20 is bonded to the first substrate 30 or the second substrate 40, so that a yield decreases and a manufacturing cost increases.
In the typical pressure sensor 10 as disclosed in Documents 1 and 2, the movable electrode 20 needs to be thinned in order to achieve a high sensitivity.
When the size of the pressure sensor 10 is reduced, the size of the movable electrode 20 is consequently reduced. In such a case, the movable electrode 20 needs to be thinned in order to obtain a sufficient displacement in response to applied pressure with the reduced size.
However, when the movable electrode 20 is thinned, the movable electrode 20 is easily drawn toward to the first substrate 30 or the second substrate 40 by an electrostatic attractive force in anodic bonding. Further, the movable electrode 20 easily receives disturbance strain due to thermal expansion of respective components caused by the change in ambient temperature, resulting in deterioration of temperature characteristics.
Accordingly, the pressure sensor in such a typical structure inherently has a limitation in improvement of sensitivity and reduction in size while maintaining temperature characteristics.